AR&T Challenges for Aerodynamics and Aeroacoustics

A total of 19 R&T Challenges were prioritized in the aerodynamics and aeroacoustics Area. Table A-1 shows the results. The R&T Challenges are listed in order of NASA priority. National priority scores are also shown.1 This appendix contains a description of each R&T Challenge, including milestones and an item-by-item justification for each score that appears in Table A-1.2

Flow interactions in the region of the propulsion–airframe interface during takeoff, climb, and cruise pose a complex design problem. Design compromises have a significant effect on the aircraft efficiency and on radiated noise. Research into improved techniques for propulsion–airframe integration would improve aircraft flexibility and performance, especially as aircraft speeds increase. To meet this objective, both computational fluid dynamics (CFD) and experimental tests are indispensable. Improvements in the accuracy of predictions of three-dimensional (3-D) steady and unsteady interactions between external and internal aerodynamics and aeroacoustics are required to enable design of future aeronautical systems. These interactions include the effects of steady and dynamic distortion on engine operations and the effects of hot, reacting exhaust flows on vehicle aerodynamics. They are particularly important in the design of vertical and short takeoff and landing (V/STOL),3 extremely short takeoff and landing (ESTOL), supersonic, and hypersonic airplanes. On V/STOL airplanes, exhaust jets are placed near the trailing edge of the wing where the aerodynamic stiffness of the high-speed flow increases wing lift by what is called the Jet Flap Effect (Spence, 1956). At supersonic speeds, adverse interactions between shock waves and boundary layers can increase drag and cause engine unstart. For many proposed hypersonic aircraft, the aircraft forebody is the inlet compression surface and the aircraft afterbody is the nozzle expansion surface, so that the airframe is part of the propulsion system. This is particularly the case with waveriders (Kuchemann, 1978). Propulsion–airframe integration has a significant impact on aircraft radiated noise. Improvements in test techniques and instrumentation are needed to characterize complex 3-D flow fields and acoustic radiation patterns. Key milestones include

Validate the predictive capablity for 3-D mean and dynamic distortion at the propulsion–airframe interface.

Validate the predictive capability of the impact of reacting exhaust flows on external aerodynamics.

The technical descriptions for the first 11 Challenges listed below contain slightly more detail than the technical descriptions for these Challenges as they appear in Chapter 3.

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VTOL airplanes can take off and land vertically. This includes tilt-rotors, the AV-8 Harrier, and the JSF, for example. VTOL airplanes do not routinely take off or land vertically because of the range-payload penalty associated with the weight limitations of purely vertical operations. Rather, they use any available field length to develop some forward motion and wing lift during takeoff to increase the useful load (fuel plus payload). They tend to land vertically only at the end of the mission, when they are lighter, after burning fuel and/or dropping weapons.STOL airplanes use high-lift systems to take off in less distance than conventional aircraft (typically a few thousand feet). Very few STOL aircraft can safely take off on runways shorter than 3,000 ft and none on runways less than 2,000 feet. (This class does not include ultralight aircraft, kit planes, etc. that can operate out of short fields due to their small size but do not have high-lift systems).ESTOL airplanes would be able to safely take off on runways of 2,000 ft. They would have high-lift systems and thrust-to-weight ratios that are higher than conventional aircraft but not as high as VTOL aircraft. ESTOL aircraft have not yet been developed for commercial or military operations.V/STOL refers to both VTOL and STOL airplanes that convert to fixed-wing flight after takeoff; it does not include helicopters.

Relevance to Strategic Objectives

Capacity (9): Novel integration of the propulsion and airframe system will be required to meet the objectives of V/STOL and ESTOL airplanes and enable general operation on shorter runways, thereby enabling a significant increase in capacity.

Safety and Reliability (3): Development of techniques to monitor conditions and predict interactions between external and internal flows will allow automated responses to potentially dangerous flight conditions, which will enhance aircraft safety.

Efficiency and Performance (9): Improved integration of the propulsion system and airframe will facilitate development of aircraft with improved performance and efficiency.

Synergies with National and Homeland Security (9): Predictive capabilities concerning engine–airframe integration will be applicable to all military aircraft.

Support to Space (9): Careful integration of the propulsion system with the airframe is required for air-breathing access-to-space vehicles.

Why NASA?

Supporting Infrastructure (3): NASA possesses both facilities and computational capabilities to investigate propulsion-airframe integration issues, but similar facilities and capabilities exist in DoD and, to a lesser extent, in industry.

Mission Alignment (9): Efforts aimed at development of an improved predictive capability that will lead to increased performance and efficiency of aircraft are very relevant to NASA’s mission.

Lack of Alternative Sponsorship (3): Propulsion–airframe integration is being investigated by DoD and industry.

Appropriate Level of Risk (9): Improving the predictive capability of propulsion–airframe integration would likely enable a new class of highly integrated aircraft. The risk associated with the development of techniques to significantly improve performance, especially with respect to novel aircraft designs, is moderate.

Aircraft performance and efficiency strongly depend on the state of the boundary layer over different portions of the wing and fuselage. Viscous drag at subsonic, supersonic, or hypersonic speeds may be reduced by developing flow control techniques that actively detect and control the state of boundary layer flows with the goal of maintaining attached flow, controlling transition to turbulence, or reducing turbulent drag. For example, takeoff and landing distances strongly depend on the ability to maintain attached flow over the wing. On a typical commercial aircraft, approximately 25 percent of the drag is due to turbulent flow over the fuselage. If a drag reduction of just 1 percent were achieved in either of these areas, a large aircraft could reduce fuel consumption by up to 100,000 gallons per year while also reducing emissions.

Flow control has the potential for significant improvements in aircraft performance when coupled with revolutionary aircraft design configurations that are designed to optimally exploit the effects of flow control. New flow control techniques include steady and unsteady flow injection, vibrating elements such as piezoelectric and voice-coil actuators, and single-dielectric barrier discharge plasma actuators that are fully electric with no moving parts. All of these techniques have shown promise in laboratory experiments and some limited-scale flight tests. However, significant work is needed to refine these approaches and develop them further in order to transition them to full-scale designs.

Accurate models are needed for the actuator effects that can be incorporated into high-fidelity numerical flow simulations. The models would also be used to optimize the design of flow actuators to improve their flow authority and expand their usable flight regime. For flow control to achieve its full potential as an element in multidisciplinary design and optimization, the accuracy of numerical simulations and models needs to be validated, and they must be computationally efficient. Key milestones include

Develop energy-efficient and flexible active flow control actuators.

Develop improved models for the operation of flow actuators.

Demonstrate techniques to incorporate these models into flow simulation schemes.

Validate models and simulation schemes through comparison with experiments.

Relevance to Strategic Objectives

Capacity (9): Flow control has the potential to improve high-lift performance and therefore reduce takeoff and landing distances, which is a critical challenge for V/STOL and ESTOL airplanes and for accommodating larger conventional airplanes on existing runways. Flow control can also increase cruise efficiency, which reduces fuel usage even as capability increases.

Safety and Reliability (3): Some flow control concepts may improve safety and reliability by improving control of flow separation in unusual flight conditions. However, other

concepts may introduce additional complexity that could have an adverse effect on reliability.

Efficiency and Performance (9): Flow control may greatly increase the efficient use of airport infrastructure through improved cruise efficiency. It also may be important for efficient supersonic flight.

Synergies with National and Homeland Security (3): Flow control may improve the fuel efficiency of military aircraft and enhance the performance of aircraft with mission constraints related to separation control (e.g., short-field performance, aft loading ramps, and highly maneuverable aircraft). Transition control may be very important for military supersonic and hypersonic vehicles.

Support to Space (3): Transition management and separation control will impact air-breathing access-to-space vehicles and may be important to reentry heat transfer and to aerodynamics of vehicles in other planetary atmospheres.

Why NASA?

Supporting Infrastructure (3): NASA possesses relevant wind tunnels, infrastructure, and computational infrastructure. The tunnels permit high-Reynolds-number testing (e.g., the National Transonic Facility) and large-scale testing (e.g., National Full Scale Aerodynamics Complex operated by DoD at NASA Ames Research Center). NASA Dryden provides full flight testing capabilities. All of them are deemed critical to this Challenge. However, while NASA’s infrastructure is very important, it is not unique in these areas.

Mission Alignment (9): This Challenge is broadly applicable to civil and military aeronautics.

Lack of Alternative Sponsorship (3): Because of the potentially high payoff, research relevant to this Challenge is conducted by many organizations outside NASA, including universities, industry, and DoD. Despite this, NASA could help coordinate these often disparate efforts and contribute directly to meeting this Challenge.

Appropriate Level of Risk (9): Despite the potential advantages of these technologies, they are not extensively used at present due mainly to a lack of understanding, tools, and validation—all areas to which NASA could contribute. This Challenge faces moderate risk.

Most classes of aircraft configurations have remained constant for many years (e.g., the tube and wing of a subsonic transport, and the main rotor plus tail rotor of a helicopter). Novel aerodynamic configurations provide substantial opportunities to make long-term breakthroughs in aircraft capabilities. A number of innovative concepts have been proposed and pursued to differing levels. Examples include the blended wing body, canard rotor wing, oblique flying wing, and strut-braced wing. A sustained research program should be promoted to develop novel aircraft configurations, including further development of existing concepts where appropriate, with emphasis on achieving breakthroughs related to the high-priority Strategic Objectives. Specific examples of potential research include novel configurations with stepwise changes in performance, such as concepts with very high cruise efficiency to reduce fuel burn and emissions; STOL aircraft, V/STOL airplanes, and high-speed rotorcraft to achieve significant changes in capacity; and quiet supersonic aircraft to improve the efficiency of the air transportation system.

Other R&T Challenges would also contribute to enabling novel aerodynamic configurations. Advances in flight mechanics and propulsion–airframe integration (R&T Challenge A1) are required to make advanced concept airplanes viable and robust. Flow control (R&T Challenge A2) could significantly enhance the capability of novel configurations, since it could be assumed a priori in the design process rather than added as an improvement to an existing airplane. Research related to the Common Theme of physics-based analysis tools is needed to move beyond empirical design tools.4 In addition, flight testing is a critical element of a successful research program in novel configurations. Key milestones include

Develop a family of aircraft configurations with cruise efficiency twice as high as conventional aircraft.

Demonstrate design approaches to develop novel configurations able to operate from small airfields.

Validate the ability to predict the performance of novel airframe configurations using data from ground and flight tests.

Relevance to Strategic Objectives

Capacity (9): Novel configurations can enable stepwise changes in aircraft speed and payload, which are the primary independent variables of capacity. Additionally, V/STOL and ESTOL concepts can increase capacity because of their ability to use smaller airports.

Safety and Reliability (3): Advanced configurations can be designed to include safety and reliability requirements up front.

Efficiency and Performance (9): Existing aircraft configurations have been refined to the limits of today’s technology. However, existing aircraft technologies can enable new

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“Physics-based” refers to the general use of scientific principles in the place of empirical data. It includes the use of principles from chemistry, biology, material science, etc.

Synergies with National and Homeland Security (3): Some novel configurations may have military applications. For example, a multirole configuration has the potential to offer military and commercial capabilities from a common platform and a common crew.

Support to Space (1): This Challenge has no impact on this Objective.

Why NASA?

Supporting Infrastructure (3): NASA wind tunnels, simulators, and flight test facilities are important elements of the infrastructure required for the development of novel configurations, but industry and DoD also have relevant infrastructure.

Lack of Alternative Sponsorship (3): NASA has an important role to play in meeting this Challenge. However, some R&T will be pursued by industry. It is important that NASA pursue collaborative opportunities, where appropriate.

Reducing the aerodynamic noise from fixed- and rotary-wing aircraft at or near airports is a long-term issue that must be addressed to increase capacity at many airports. Design tools are needed at both the technology level and the aircraft system level, with particular attention to integrated solutions for aerodynamic and operational issues. This Challenge requires a balanced combination of physics modeling, tool development, and experiments.

For large fixed-wing transports, airframe noise on approach has become important, requiring efficient aerodynamic designs for flaps, ailerons, and landing gear that minimize noise radiation. Examples include smoothly varying section- and spanwise profiles to obtain high lift and low noise and high-drag/low-noise devices, which permit steep approaches, mitigating noise on the ground. Key research needs include a basic understanding of the fluid physics of cavity-like flows, unsteady flow–solid surface interactions, flow separation, development of physics-based source noise prediction methods, and development of improved computational aeroacoustic tools.

Many of today’s airports now limit operations because of the noise emitted to the surrounding community. Future passenger growth at many airports will be limited if the noise levels emitted by the newer aircraft are not reduced further, thus adversely affecting capacity. Off-loading the main runway of regional jets by using ESTOL aircraft and rotorcraft, thus reducing congestion for larger passenger aircraft on the main runway, will dramatically increase capacity by allowing more takeoffs and landings at existing airports without increasing demand for runway usage (NRC, 2003; FAA, 2000). However, it will only be possible if these ESTOL aircraft and rotorcraft are quiet. To reduce takeoff and landing noise, such aircraft require the development of very high lift devices that are quiet and do not impose undue performance sacrifices Novel technologies are needed to decrease unsteady interactions between the propulsive lift devices and the lifting and control surfaces of the aircraft. These aircraft should also be designed to shield major sources of noise from the ground.

Minimizing impulsive noise generated by rotorcraft requires a better understanding of the aerodynamic state of the rotor, which is a strong function of the rotor blade structural properties. Both main and tail rotor noise are important, depending upon the configuration chosen. Methods of noise reduction include lowering the rotor rpm (which can degrade performance), reducing the major disturbances (shedding of tip vortices) to the following blades though rotor design and/ or vortex-blade position control, integrating advanced control schemes for active rotorcraft noise reduction, and reducing the rotor response to vortex-induced disturbances. Also required are advances is the ability to predict rotor dynamic stall, to predict wake vortex dynamics, and to design rotor blades that minimize the time derivative of the blade’s aerodynamic loading response to sharp-edged disturbances. Key milestones include

Improve techniques for prediction and control of the aeroacoustics associated with high-lift devices, protu-berances, and cavities for fixed-wing aircraft.

Develop techniques for the prediction and design of quiet drag devices for fixed-wing aircraft.

Demonstrate novel rotor system design tools that can be used to reduce rotor noise with minimum performance sacrifices for rotorcraft.

Relevance to Strategic Objectives

Capacity (9): Reducing aerodynamic noise of both fixed-and rotary-wing aircraft increases capacity by allowing more flights in and out of noise-impacted airports and by facilitating expansion of flight operations at satellite airports.

Safety and Reliability (1): This Challenge has little or no impact on this Objective.

Efficiency and Performance (3): Reducing noise can in some cases also improve aircraft performance. In a more

Synergies with National and Homeland Security (3): Noise reduction is important to make military aircraft more difficult to detect and to make operations near noise-sensitive areas more acceptable to the public.

Support to Space (1): This Challenge has no impact on this Objective.

Why NASA?

Supporting Infrastructure (3): Although NASA does not own all the nation’s best aeroacoustic facilities, it does have access to a unique aeroacoustic facility for rotor noise testing: the 40 × 80 × 120-foot acoustically treated tunnel whose size allows far-field acoustic measurements of medium- and large-scale rotorcraft. However, this tunnel is now operated by the U.S. Air Force.

Mission Alignment (9): This Challenge is very relevant to NASA’s mission.

Lack of Alternative Sponsorship (3): Industry, the FAA, and the DoD also carry out and sponsor work related to this Challenge.

Appropriate Level of Risk (9): This Challenge faces high risk because most of the easily attained techniques have already been incorporated into aircraft design.

The aerospace industry lacks computational analysis and design tools that can rapidly and accurately predict complex flow behavior driven by boundary layer transition, flow separation, novel configurations, off-design operation, and multidisciplinary interactions. To meet this need, physics-based design tools must be developed and systematically validated in representative environments. Ideally, they should have the following attributes:

Ability to fully describe the state of the fluid at any point in the solution domain, with useful information on the surfaces.

Inverse design capability.

The benefit of technologies developed by this Challenge would be enhanced by the parallel development of multidisciplinary design tools to address complex nonlinear interactions and of methods to handle parameter uncertainties in a computationally efficient way (see Challenge A11). Key milestones include

Develop improved techniques for the prediction of boundary layer transition on 3-D configurations and validate them against ground and flight test data.

Synergy with National and Homeland Defense (3): Improvements in aerodynamic prediction capabilities are expected to improve DoD aircraft development.

Support to Space (3): Boundary layer transitional behavior through multiple speed regimes remains one of the large uncertainties for air-breathing access-to-space vehicles. Improvements in predictive capabilities are likely to improve the potential performance of these vehicles by reducing design margins.

Why NASA?

Supporting Infrastructure (9): NASA possesses some of the most advanced computational fluids modeling and empirical validation tools in the world, across multiple speed regimes and environments.

Mission Alignment (9): NASA’s historical CFD code development provides a foundation for much of industry’s design codes today; this Challenge is very relevant to NASA’s mission.

Adverse weather conditions, including storms and icing conditions, significantly reduce the capacity and reliability of the air transportation system. Adverse weather also degrades system safety. This issue is of importance to both civil and military aviation. Research is needed to improve the ability to predict and monitor environmental conditions and develop aerodynamic designs and techniques that are robust to adverse conditions.

At present, wind-shear warning systems are built into commercial aircraft, icing hazards are handled by regulatory constraints on flight operations, and prediction techniques are largely empirical. Low-cost techniques to measure environmental conditions ahead of an aircraft should be developed. Examples of promising techniques include microwave, lidar, and laser-acoustic measurement techniques. Efforts to miniaturize and reduce the cost of the measurement equipment should be supported. Techniques to predict and mitigate the impact of adverse environmental conditions on the aircraft operation should be improved. Required improvements include the development of models to predict the impact of multiphase, nonequilibrium situations encountered under icing conditions; validation of icing prediction capabilities to enable a reduction in the high cost of aircraft and helicopter icing certification; and models for the complex-flow, time-dependent, 3-D interactions encountered during wind shear or ambient turbulence on the aircraft flowfield. Key milestones include

Develop high-bandwidth techniques to respond to and mitigate the impact of upstream environmental conditions.

Relevance to Strategic Objectives

Capacity (9): Improving operations in adverse weather conditions will increase capacity by allowing more on-time flights and fewer diversions to other airports.

Safety and Reliability (9): Identifying and mitigating adverse environmental conditions will reduce accident rates and increase the reliability of the air transportation system.

Efficiency and Performance (3): Through mitigating the adverse impacts of weather, air transportation resources can be optimally used with fewer operational constraints.

Energy and Environment (1): This Challenge has no impact on this Objective.

Synergies with National and Homeland Defense (9): DoD and DHS operations will be enhanced if adverse weather conditions become less of a constraint. In particular, DoD capabilities are substantially enhanced if U.S. military forces can operate effectively in weather conditions that degrade the effectiveness of enemy forces, and if enemy forces cannot use adverse weather as cover.

Support to Space (1): Most space operations have the luxury of waiting for favorable environmental conditions. This Challenge has little impact on this Objective.

Formation flight is currently used by military airplanes for a variety of operational reasons, although rarely for drag reduction. Recent breakthroughs in accurate navigation and control make possible extended precision formation flight in cruise and permit exploiting favorable interference to reduce vortex drag. Although this phenomenon is well known, the magnitude of the potential savings is not widely appreciated. Three airplanes flying in formation and designed to best exploit these effects could reduce vortex drag by more than 50 percent in cruise, a greater reduction than that obtainable by extensive laminar flow control on the wing. This would mean roughly a 20 percent reduction in total drag under identical operating condition. However, with less induced drag, the optimum altitude increases, reducing viscous drag as well. The net result is almost a 30 percent reduction in total drag. Unlike the tight formations required for military applications, drag savings are possible even with longitudinal separations of several miles (Spalart, 1998), reducing safety concerns associated with formation flight. Initial NASA work on autonomous formation flight has identified some of the technology requirements for achieving these savings, but considerable research remains in both control methodology and aerodynamic design to take most advantage of the concept. Applications to cargo airplanes, rotorcraft, and even supersonic flight are possible but have not been studied extensively. Aerodynamic challenges include vortex location

Relevance to Strategic Objectives

Capacity (3): Formation flying may have some effect on capacity through reduced takeoff weight, which will lead to reduced spacing requirements and fewer noise-related restrictions. Formation flying will also increase en route density, leading to increased capacity.

Safety and Reliability (1): Formation flying introduces additional safety concerns, some of which are ameliorated with large longitudinal spacing.

Efficiency and Performance (9): Reduced fuel consumption and the potential for more efficient cargo delivery systems will improve efficiency and performance.

Wingtip vortices produced by airplanes present a danger to following aircraft, so airplane designs and techniques that mitigate the strength of these vortices, techniques to locate and determine their strength, and techniques to predict their propagation and decay are important factors in minimizing aircraft separation and enhancing safety.5 (Since aircraft lift is intimately tied to the production of circulation, these vortices cannot be completely eliminated.) Currently, aircraft separation standards are set by conservative estimates of the wake vortex trajectory (generally a sinking trajectory, but also affected by local weather conditions) and decay rate. Techniques to measure the characteristics of upstream wake vortices include lidar and laser-acoustic techniques, but these technologies are currently expensive (limiting their use to larger aircraft) and are less reliable than desired. Research into techniques to predict the formation, trajectory, and decay of vortices needs to be performed. Key milestones include

Relevance to Strategic Objectives

Safety and Reliability (9): An improved understanding of wake vortex dynamics will improve safety of following aircraft. Techniques aimed at minimizing the strength or increasing the dissipation rate will present weaker vortices to following aircraft.

Efficiency and Performance (3): Technologies aimed at reducing aircraft spacing will improve the efficiency and performance of the air transportation system.

Energy and Environment (1): This Challenge has no impact on this Objective.

Synergy with National and Homeland Defense (1): This Challenge has minimal impact on this Objective.

Support to Space (1): This Challenge has no impact on this Objective.

Why NASA?

Supporting Infrastructure (3): NASA and the aircraft industry have conducted research into wake physics modeling

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The scope of this Challenge does not include and would not directly apply to helicopter blade wakes.

and measurement; both have important infrastructure to bring to this area.

Mission Alignment (9): This Challenge is very relevant to NASA’s mission.

Lack of Alternative Sponsorship (3): Relevant research conducted by NASA is synergistic and closely aligned with similar work of the FAA.

Appropriate Level of Risk (3): Good progress can be expected in this area within the next decade.

A9 Aerodynamic performance for V/STOL and ESTOL,including adequate control power

Since 2001, the U.S. aviation industry has undergone a profound change. On many routes, regional jets have replaced propeller-driven aircraft, which used different runways and flew at lower altitudes than the large commercial transports. Regional jets are using the same runways and airways as the large transports. This increases congestion and delays at major airports, which degrades the performance of the entire air transportation system.

Powered lift (V/STOL and ESTOL) airplanes and rotorcraft may provide solutions to this problem. V/STOL jets with highly swept wings can operate from the short runways previously used by straight-wing propeller-driven transports. These aircraft, together with rotorcraft, may also be able to operate from taxiways and other paved areas at major airports or smaller regional airports. Any of these applications would relieve congestion on the main runways at major airports. In responding to natural disasters and carrying out military operations, low-cost VTOL tactical transports would be able to operate from short, austere landing fields near the focus of attention (e.g., the location of injured civilians or troops, battle areas, and landslides).

These aircraft will require advances in aerodynamics; propulsion; acoustics; stability and control; structures and materials; and guidance, navigation, and communications. Specific aerodynamic issues that require attention include development of a low-drag, high-lift system, simple boundary layer control systems to prevent wing leading-edge separation, systems to provide pitch trim and control power at low speeds, a reversing deflecting exhaust nozzle, and wing design and fuselage shaping to reduce cruise drag in the transonic regime.

An important task for research related to rotorcraft and fixed-wing VTOL aircraft is improving hovering and cruise efficiency. Reductions in downward forces in near-hovering flight dramatically improve the payload capability of tilt-rotor and powered-lift aircraft. Active control of large separation regions on these aircraft through blowing, zero-mass effectors and integrated mechanical devices are promising methods of reducing download. Active twist control of the rotor also allows the rotorcraft to be designed to better match the hover and cruise design conditions, thereby improving efficiency. Active control of separation regions and smart design guided by high-fidelity codes will decrease cruise drag and greatly improve the performance of V/STOL airplanes and rotorcraft. Validated codes require interdisciplinary research efforts as well as efforts to improve separation prediction and control. Key milestones include

Develop low-drag, high-lift systems.

Demonstrate systems to provide pitch trim and control power at low speeds.

Relevance to Strategic Objectives

Capacity (9): This Challenge could greatly increase capacity by shifting regional jets from the major runways to smaller runways and/or taxiways and by enabling the use of smaller regional airports.

Safety and Reliability (3): Developing adequate control power at high lift, low speed would increase safety.

Efficiency and Performance (3): Novel methods of improving the performance of V/STOL airplanes (e.g., download reduction and avoiding flow separation regions in cruise) and rotorcraft can improve their efficiency.

Energy and Environment (1): This Challenge has no impact on this Objective.

Synergies with National and Homeland Security (3): Mobility, especially over unprepared or short fields, is very important for quick-response situations. V/STOL and ESTOL airplanes would enhance these capabilities.

Support to Space (1): This Challenge has no impact on this Objective.

Why NASA?

Supporting Infrastructure (3): Large-scale testing is critical to this Challenge. The best large-scale ground testing facility is the 40 × 80 × 120 ft tunnel at NASA Ames. NASA has access to this facility, although it is now operated by the U.S. Air Force. NASA also has smaller scale facilities that can support this Challenge.

Mission Alignment (9): This Challenge capitalizes on NASA in-house expertise in powered lift and rotorcraft development and is very relevant to NASA’s mission.

Lack of Alternative Sponsorship (3): Industry and DoD also carry out and sponsor work related to this Challenge.

Safe, efficient, cost-effective, environmentally acceptable supersonic flight over land remains elusive nearly 60 years after airplanes broke the sound barrier. The principal remaining problems are sonic boom mitigation, public acceptance, and sustained supersonic flight performance. Today, federal regulations prohibit civil supersonic flight over land. If this regulatory barrier can be overcome, it will probably stimulate investment that would overcome the other barriers and help usher in a new era of time-critical air travel. Building on the recent in-flight validation of NASA’s shaped sonic boom persistence theory, a robust and comprehensive plan of research for technology maturation and tool development should be pursued to determine if practical supersonic airplanes can be developed whose sonic boom is acceptable to the public (Pawlowski et al., 2005). Such a plan should comprise the determination of what level of sonic boom is acceptable to the public; community exposure testing; aircraft shaping techniques that result in low-amplitude, acceptable acoustic signature with minimal performance impact; critical propulsion–airframe integration technologies commensurate with low-boom design; aircraft and acoustic scaling methodologies; sensitivities to off-design conditions under a variety of atmospheric conditions; rapid and inverse computational design tools that address multiple design constraints; systematic validation through ground and flight test; and metrics to assess progress and guide continuation according to the plan. This Challenge is closely tied to Challenge B8. Key milestones include

Develop guidelines for allowable exposure of the public to sonic booms.

Develop accurate techniques for the prediction of sonic boom propagation through the atmosphere under realistic environmental conditions.

Demonstrate novel aircraft shapes that minimize sonic boom levels.

Relevance to Strategic Objectives

Capacity (3): Enabling supersonic flight over land will increase capacity by moving airplanes through the system more rapidly, although, at least initially, such a capability will affect only a small segment of the population.

Safety and Reliability (1): This Challenge has no impact on this Objective.

Efficiency and Performance (3): Success in this Challenge will usher in a new era of time-critical travel.

Energy and Environment (9): This Challenge will profoundly reduce the noise produced by supersonic airplanes.

Synergy with National and Homeland Defense (3): This Challenge will enable quiet supersonic airplanes, which will also benefit military missions.

A11 Robust and efficient multidisciplinary design tools

Multidisciplinary design tools are pervasive in aeronautics. A multidisciplinary, integrated system-level design approach to assessing potential costs, benefits, and risks would help advance aerodynamic technologies, shorten the design cycle time for conventional aircraft, and develop novel aircraft configurations. Tools that couple a small number of disciplines have reached a level of maturity and fidelity that should be exploited by design tools. For example, aeroelastic design tools are now within reach that couple CFD and finite-element analyses for full aircraft configurations. More recently, multidisciplinary design tools have begun to incorporate a broader range of disciplines, and techniques such as multidisciplinary design optimization (MDO) have been used to a limited extent in aircraft conceptual design.

One of the major limitations of past efforts to create MDO tools has been a low level of fidelity, driven by a lack of physics-based models that are sufficiently efficient for use at the system design level. In addition, MDO tools have often been developed and applied for very specific applications and flight conditions, so they lack flexibility. Key challenges associated with next-generation multidisciplinary design tools include tool fidelity, computational efficiency, and the ability to handle parameter uncertainties. The practical resolution of these challenges will require fundamental research efforts in physics-based models for use in design tools (see R&T Challenge A4b), new design methodologies that can seamlessly manage models of multiple fidelities for the various components of the system, methods to increase the computational efficiency of tools, methods to handle complex interactions with high accuracy, and automated techniques for handling and propagating parameter uncertainties throughout the design. Key milestones include

Energy and Environment (9): The novel designs that could be achieved with these tools could have a significant impact on energy and environmental issues.

Synergies with National and Homeland Defense (3): Multidisciplinary design tools are applicable to military aircraft.

Support to Space (3): Multidisciplinary design tools could also be used to improve space vehicle design.

Why NASA?

Supporting Infrastructure (3): NASA has a strong track record of R&T in system design tools and multidisciplinary design optimization. NASA has relevant computational infrastructure, although it is not unique.

Mission Alignment (9): This Challenge is very relevant to NASA’s mission.

Lack of Alternative Sponsorship (3): Both industry and other government agencies are pursuing work in this Challenge, although NASA has a unique opportunity to provide a bridge between academic research and industrial needs.

Appropriate Level of Risk (3): This Challenge faces low risk.

A12 Accurate predictions of thermal balance and techniquesfor the reduction of heat transfer to hypersonic vehicles

Air-breathing access-to-space and reentry vehicles must operate in a stressing aerothermodynamic environment that requires high-performance, robust thermal protection systems (TPSs). The cost and feasibility of air-breathing launch vehicles are extremely sensitive to mass. Therefore, more accurate techniques are needed to (1) predict aerothermal loads (and thus decrease the design margins associated with the TPS) and (2) minimize both local and integrated heat transfer to the vehicle, which can significantly increase system performance. Specific needs include improved predictions of how the following factors affect heat transfer in the hypersonic environment: ablation, boundary layer transition, highly cooled walls (which affects boundary layer turbulence), wall chemistry (including catalytic effects), and radiating shock layers.

Relevant techniques include novel aerodynamic shaping, active flow control, transpiration cooling, and emissivity control. Research is also required to determine the utility of plasma aerodynamic and magnetohydrodynamic flow manipulation for heat transfer reduction. Key milestones include

Improve models for predicting the effects of ablation on heat transfer.

Relevance to Strategic Objectives

Capacity (1): This Challenge has little impact on this Objective.

Safety and Reliability (1): This Challenge has little impact on this Objective.

Efficiency and Performance (3): Development of accurate heat transfer prediction techniques and mitigation of local high-heat-transfer regions will improve the performance of supersonic and hypersonic vehicles.

Energy and Environment (1): This Challenge is principally aimed at hypersonic vehicle applications, so this Challenge has no impact on this Objective.

Synergies with National and Homeland Defense (9): Development of robust supersonic and hypersonic systems can help address DoD missions in areas such as missile defense, time-critical strike, prompt global strike, and access to space.

Support to Space (9): Heat transfer is an important issue associated with air-breathing access-to-space and reentry systems.

Air-breathing access-to-space vehicles capable of horizontal takeoff and landing hold significant promise in providing low-cost access to space. Mission and operational flexibility is greatly enhanced by the ability to operate from sites similar to those utilized for conventional aircraft. However, factors such as high wing sweep, sharp leading edges, and use of a propulsion system designed for hypersonic flight significantly increase runway length and require greatly modified flight corridors relative to conventional aircraft.

Research in this area should include, but not be limited to, development and evaluation of high-lift systems, active and passive flow control, consideration of two-stage-to-orbit configurations, optimum fuselage and wing shaping, morphing structures, and configurations with enhanced propulsion–airframe integration for improved low-speed flight characteristics. Strategies to improve takeoff and landing performance without significantly impacting payload capacity, range, fuel, and structural weight and cost are vital because the overall efficiency and cost of space launch vehicles is very sensitive to weight. More accurate tools are needed to predict the effects of flow control, vehicle shaping, and propulsion–airframe integration techniques on boundary layer behavior and flow separation. In addition, integrated system analysis tools must be developed and validated to predict how modifications to improve takeoff and landing performance will affect vehicle performance throughout the flight profile. The aerodynamic tools and novel strategies for improving low-speed performance must be validated by relevant ground and flight tests. Key milestones include

Validate predictive capability for integrated vehicle aerodynamics in the presence of the runway.

Relevance to Strategic Objectives

Capacity (1): This Challenge will have no impact on this Objective in the time frame considered.

Safety and Reliability (3): Some of the flow control and separation mitigation techniques that address this Challenge may be applicable to subsonic commercial aircraft low-altitude flight and provide improvement in safety and reliability during the takeoff and landing phases of flight.

Efficiency and Performance (1): This Challenge has little to no impact on this Objective.

Energy and Environment (1): This Challenge has little to no impact on this Objective.

Synergies with National and Homeland Security (3): Some of the flow control and separation mitigation techniques that address this Challenge may be applicable to current and future high-speed military aircraft.

Support to Space (9): This Challenge could significantly improve the operational flexibility of access-to-space missions.

Why NASA?

Supporting Infrastructure (3): NASA possesses wind tunnels that are well suited for this Challenge. NASA expertise and computational facilities are appropriate but not unique.

Mission Alignment (9): This Challenge is very relevant to NASA’s space exploration mission.

Lack of Alternative Sponsorship (9): Horizontal takeoff and landing vehicles will not be developed without federal investment, and the fundamental physics studies on boundary layers and separation are appropriate long-term research efforts for NASA. No significant, sustained work is done in this area by other government entities or industry.

Appropriate Level of Risk (9): This Challenge faces high risk.

A14 Efficient control authority of advancedconfigurations to permit robust operations athypersonic speeds and for access-to-space vehicles

Hypersonic vehicles have the potential to provide affordable access to space, safe and predictable entry from space, flight in other planetary atmospheres, prompt global reach, and missile defense. Aerodynamic configurations optimized for hypersonic cruise or acceleration present significant design challenges. They can prove inefficient at off-design conditions, their performance is affected by interactions with the aerodynamic flow through the engine flow path, and they frequently require control at high altitudes.

Control of vehicles operating in this regime will require a better physics-based understanding of the flow field characteristics. The characterization and prediction of the effect of boundary layer transition on aerodynamic configurations remains a significant challenge. More accurate knowledge of the state of the boundary layer, transition location, areas of separation, and certain viscous interactions, including shock-wave–boundary-layer interaction, will facilitate development of configurations with adequate control authority. The implementation of flow control concepts through the use of novel actuation (e.g., plasma/magnetohydrodynamic concepts) may prove useful. In addition, characterizing the salient physics of improved measurement techniques and the

Relevance to Strategic Objectives

Safety and Reliability (1): This Challenge has no impact on this Objective.

Efficiency and Performance (3): Research in this Challenge will extend the understanding of complex fluid physics, which will increase understanding at lower speeds as well.

Energy and Environment (1): This Challenge has no impact on this Objective.

Synergies with National and Homeland Security (9): Most high-speed applications will benefit from the improved control authority techniques investigated by this Challenge.

Support to Space (9): This Challenge provides a significant contribution to space exploration and access-to-space missions.

Why NASA?

Supporting Infrastructure (3): NASA possesses some relevant capabilities in the areas of hypersonic aerodynamics, but several important assets exist at DoD as well.

Mission Alignment (9): Hypersonic aerodynamics is a fundamental enabling technology that is very relevant to NASA’s mission.

Lack of Alternative Sponsorship (3): Both NASA and DoD conduct research associated with improvements in stability and control of hypersonic vehicles to better understand and control vehicle flight dynamics.

Appropriate Level of Risk (9): This Challenge faces high risk.

A15 Decelerator technology for planetary entry

Effective and reliable decelerator technologies for Earth reentry and planetary entry are needed to support NASA’s space exploration mission. Such technologies must yield acceptable deceleration loads (which are most stringent for crewed vehicles). In addition, some missions require the generation of lift for improved cross-range and control of the entry trajectory. The key task for this Challenge is to provide the required aerodynamic loads in a robust and reliable system while yielding efficient, low-mass thermal protection.

Relevant research includes characterization of planetary atmospheric conditions and chemistry; development and evaluation of optimum vehicle shapes and novel configurations; use of parachutes, parafoils, ballutes; active and passive flow control; and accurate prediction of aerodynamic and thermal loads and trajectories during aerocapture and aero-assisted orbital transfer operations. Improved accuracy is needed in tools designed to predict thermal loads in the planetary atmosphere under consideration and to predict unsteady aeroelastic effects on structures that are highly flexible or whose shape may vary (e.g., due to ablation). In addition, integrated system analysis tools are also needed. Validation of the aerodynamic tools and novel strategies for improving performance and reliability of decelerator technologies requires ground and flight testing. Key milestones include

Develop integrated system analysis tools for planetary entry system design.

Relevance to Strategic Objectives

Capacity (1): This Challenge has no impact on this Objective.

Safety and Reliability (1): This Challenge has no impact on this Objective.

Efficiency and Performance (1): This Challenge has no impact on this Objective.

Energy and Environment (1): This Challenge has no impact on this Objective.

Synergies with National and Homeland Security (3): Ballistic and hypersonic cruise missiles will derive some benefit from the thermal protection elements of this Challenge.

Support to Space (9): This Challenge significantly contributes to space exploration and access-to-space missions.

Why NASA?

Supporting Infrastructure (3): NASA possesses relevant and unique high-enthalpy tunnels and reacting flow expertise. Other tunnels, materials testing, and computational facilities needed for this topic also exist elsewhere (e.g., the DoD facilities at the Arnold Engineering and Development Center and the Calspan–University of Buffalo Research Center).

Mission Alignment (9): The Challenge is very relevant to NASA’s space exploration mission.

A16 Low-Reynolds-number and unsteadyaerodynamics for small UAVs

This Challenge deals with the special aerodynamic issues associated with small UAVs (wing spans on the order of 6 inches) that are capable of high maneuverability and flight in confined spaces. These vehicles are of interest to DoD and DHS for missions that involve autonomous reconnaissance in urban areas, including inside buildings. The vehicles are also relevant to flight in the martian atmosphere. Prevalent concepts include flapping wings that mimic birds or insects and rotating, lifting rotors. The dominant aerodynamics for these vehicles involves highly unsteady, dynamic-stall, vortex-driven flows. The lifting capability of state-of-the-art flapping-wing vehicles is too low. This Challenge seeks to enhance dynamic lift through different concepts that might involve reflexive wing structures, flow-energy extraction, and active flow control. This will require physics-based flow models and time-resolved experiments for highly unsteady flows to maximize lift, maneuverability, and flight control of these vehicles. Key milestones include

Airships capable of operating in the stratosphere for extended periods of time are being investigated for communications relay and surveillance applications. They offer the ability to provide wide area coverage from a persistent platform, while enabling economical retrieval of payloads for repair or replacement. These vehicles build on technologies available for existing airships that operate at lower altitudes, but require advances in lightweight hull fabrics, efficient energy generation and storage, low-drag aerodynamic configurations, and efficient propulsion systems capable of operation in a low-Reynolds-number environment.

Minimizing the drag of these vehicles is important, since a significant portion of the required onboard energy is expended by the propulsion system for station-keeping against winds. Aerodynamic issues of interest include boundary layer transition prediction for flexible thin-wall hulls, prediction of viscous drag in the regions of turbulent flow, boundary layer separation control, and unsteady aerodynamics associated with gossamer structures. Key milestones include

Synergies with National and Homeland Security (9): The development of efficient and affordable stratospheric airships will provide a new class of vehicle for providing persistent surveillance and communication relay. Both DoD and DHS are investigating high-altitude airships for these applications.

Support to Space (1): This Challenge has no impact on this Objective.

Why NASA?

Supporting Infrastructure (3): NASA has both computational and experimental tools relevant to the development of airship technology. DoD and industry are capable of contributing infrastructure as well.

Mission Alignment (3): Development of fundamental technologies for gossamer vehicles is very relevant to NASA’s mission.

Lack of Alternative Sponsorship (3): Both DoD and DHS are currently investing in the relevant technologies.

Appropriate Level of Risk (9): This Challenge faces high risk, but significant progress in understanding the underlying limitations of the technology could be made within the next 10 years.

Vehicles returning from space through the atmosphere will encounter regions where communication is greatly reduced and at times nonexistent due to interaction with the atmosphere. At hypersonic velocities, aerothermal stresses create a charged flow field around the vehicle, eventually eliminating communication through this highly charged shear layer. Maintaining continuous communication with these vehicles is important for accurate control, targeting, and continuous health monitoring. As hypersonic concepts move to flight experimentation, robust communication with test vehicles will be increasingly important for the test range to have adequate control, destruct authority, and capability to download sufficient data throughout the flight regime.

Research is needed to understand and characterize the shear and boundary layers, and the relationship between signal transmission and the flow physics. Several promising concepts are under investigation for minimizing the effects of this charged flow field, creating innovative designs and providing information through test and analysis.

Currently, communication blackout is tolerated and systems have been designed around this problem by accepting a ballistic trajectory until communication is restored. Several technologies have been demonstrated in limited capacity on the ground and in flight but not one has completely alleviated the problem. Key milestones include

Why NASA?

Mission Alignment (9): This Challenge is very relevant to NASA’s mission.

Lack of Alternative Sponsorship (3): Both NASA and DoD conduct research associated with improvements in reentry physics.

Appropriate Level of Risk (3): This Challenge faces very high risk.

A19 Aircraft protective countermeasures based on arange of small deployed air vehicles

This Challenge deals with the special aerodynamic issues associated with small subsonic or supersonic flight vehicles that might be deployed from commercial aircraft as a countermeasure to an attack from ground- or aircraft-launched missiles. The vehicles need to be highly maneuverable, autonomous, able to station-keep long enough to allow the passenger aircraft to escape the airspace, and inexpensive enough to be adopted by a wide range of civil aircraft. Relevant technologies include flow control approaches for flight control in subsonic (and, perhaps, supersonic) regimes. Possible vehicle configurations include small missile bodies and miniature delta-wing aircraft. Achieving maximum flight control will be critical and may involve advanced flow controls that manipulate coherent vortices and shock waves to produce large asymmetric surface pressure loading and resultant force vectoring. Key milestones include

National Research Council (NRC). 2003. Securing the Future of U.S. Air Transportation: A System in Peril. Washington, D.C.: The National Academies Press. Available online at <http://fermat.nap.edu/catalog/10815.html>.

The U.S. air transportation system is very important for our economic well-being and national security. The nation is also the global leader in civil and military aeronautics, a position that needs to be maintained to help assure a strong future for the domestic and international air transportation system. Strong action is needed, however, to ensure that leadership role continues. To that end, the Congress and NASA requested the NRC to undertake a decadal survey of civil aeronautics research and technology (R&T) priorities that would help NASA fulfill its responsibility to preserve U.S. leadership in aeronautics technology. This report presents a set of strategic objectives for the next decade of R&T. It provides a set of high-priority R&T challenges—-characterized by five common themes—-for both NASA and non-NASA researchers, and an analysis of key barriers that must be overcome to reach the strategic objectives. The report also notes the importance of synergies between civil aeronautics R&T objectives and those of national security.

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